Very long burn duration hybrid combustor

ABSTRACT

A long burning hybrid combustor is described wherein an oxidizer injector is mounted within a centrally extending port in a fuel grain, the injector outlet being in proximity to the truncated end of a concave conical surface formed by a cavity in the downstream end of the fuel grain. The downstream end of the fuel grain is restrained and constant force means are employed to drive the grain in a downstream direction as the grain is burned. Ignition means comprising a coaxially extending tube, mounted within the oxidizer injector, supplying a gaseous fuel to form a combustible mixture at the outlet of the injector and means for forming a spark at the outlet are also described.

D United States Patent [191 [111 3 709 652 9 9 Iwanciow et al. 1 Jan. 9,1973 {54] VERY LONG BURN DURATION 3.156.092 |1/|964 Holzmnn 1. ..6u/:s|HYBRID COMBUSTOR 3,325,998 6/1967 Novotnyl 1 1 ..oO/l5l 3,358,45212/1967 Ehrenfed et a1 ..oO/229 lnvemorsr Bernard lwaflciow, Sunnyvale;3,456,440 6/1969 Keller ..60/25l Allen L. Holzman, Palo Alto, both ofCalif- Primary Examiner-Samuel Feinberg [73] Assignee: United AircraftCorporation, East Attorney-"Steven Stone Hartford, Conn. ABSTRACT [22]Flled' May 1969 A long burning hybrid combustor is described wherein[21] Appl. No.: 825,886 an oxidizer injector is mounted within acentrally extending port in a fuel grain, the injector outlet being inproximity to the truncated end of a concave conical ..431/9F62,l6k0g;5)(1) Surface formed by a cavity in the downstream end of 58 60 251 thefuel grain. The downstream end of the fuel grain is 1 0 care restrainedand constant force means are employed to drive the grain in a downstreamdirection as the grain I56] 1 References is burned. Ignition meanscomprising a coaxially ex- UNITED STATES PATENTS tending tube, mountedwithin the oxidizer injector,

supplying a gaseous fuel to form a combustible mix- 2,921,52l 1/1960 LaHaye et a1. ..l02/39 mm at the outlet of the injector and means forform- 2 ,945,343 7/1960 Gongwer 2,987,881 6/1961 3,128,599 4/1964 ing aspark at the outlet are also described.

8 Claims, 2 Drawing Figures OXIDIZER FUEL HIGH /24 VERY LONG BURNDURATION HYBRID COMBUSTOR BACKGROUND OF THE INVENTION pressurizingmedium, the generation of an intense source of radiant energy usable forillumination or as targets for homing missiles, for example. Variousattempts have been made to utilize rocket motors for such non-propulsionfunctions. Hybrid combustors employing a solid fuel and a fluid oxidizeror vice versa would appear particularly suited for such use since theyare relatively simple in construction and control, can employ componentsof relatively long shelf life and can be started and stopped repeatedly.

Unfortunately, the requirements for an effective rocket motor areantithetical to those of an efficient combustor designed for purposesother than producing reaction thrust. For example, rocket motors burnthe fuel and oxidizer at relatively high consumption rates within acombustion chamber and exhaust the products through a sonic (choked)orifice or nozzle to produce thrust. Thus, the reacting materials areconsumed at a far greater rate than is necessary for a non-propulsiondevice; and if radiant energy is a desired output, a large portion ofthe radiant energy generated is unavailable since it is not transmittedthrough the combustion chamber.

For example, a combustor for non-propulsion use could be required toburn for 30 minutes or longer and such durations are not normallyobtained in a conventional rocket motor. Further, for manynon-propulsion uses it is neither necessary nor desirable to operate atthe high combustion chamber pressures required when thrust is needed.

A typical hybrid rocket motor employs a hollow cylindrical fuel grainwhich burns at the surface of the central port. Thus, the burn time is afunction of the wall or web thickness of fuel grain rather than thelength and the O/F ratio may vary as the web is consumed. Attempts toproduce an end-burning hybrid in which the burn time would beproportional to length have not proved satisfactory since the locationof the burning surface with respect to the fluid injector or the outletof the device or both will vary as the solid grain is consumed, therebychanging the operating characteristics of the device in a complicatedmanner. According to this invention, however, a long burn durationhybrid combustor is provided which operates at substantially ambientconditions while automatically maintaining the location of the burningsurface with respect to the injector and the outlet throughout the burnperiod.

It is, accordingly, an object of this invention to provide a longburning hybrid combustor.

It is another object of this invention to provide a long burning hybridcombustor which automatically maintains the location of the burningsurface of the solid component with respect to the fluid injector andthe outlet of the combustor.

It is another object of this invention to provide a nozzleless hybridcombustor.

It is another object of this invention to provide a hybrid combustorwherein the O/F may be maintained constant.

These and other objects of this invention will be readily apparent byreference to the accompanying drawing wherein:

FIG. 1 is a schematic representation of a foreshortened side view,partly in section, of combustor according to this invention; and

FIG. 2 is a foreshortened perspective view of another embodiment of thesolid grain of FIG. 1 showing its initial configuration.

A DESCRIPTION OF THE INVENTION Referring now to FIG. 1, a nozzlelesslong duration hybrid combustor is shown generally as 1. As used herein,a combustor is considered to be nozzleless if there is no choked orsonic orifice located between the situs of combustion and theenvironment. The combustor 1 comprises a fixed grain support 2 and afluid manifold 3 maintained in fixed spatial relationship by supportmeans 4 or other suitable support means. The fixed grain support 2 isprovided with a central cylindrical passage adapted to slidably receivethe fuel grain 5. The upstream portion of this passage may be providedwith a slight outward taper shown at 6 to facilitate insertion of grain5 and the outlet or downstream portion of member 2 is provided with aninward taperto form lip 7 which restrain the motion of grain 5 as willbe more fully described below.

Grain 5 is in the form of a hollow cylinder, the downstream portion ofwhich is initially configured with a cavity to provide a concavetruncated conical surface on the downstream end. The upstream end mayterminate as shown in FIG. 1 or optionally, to provide for totalconsumption of grain 5 in the operation, the exterior surface of theupstream portion of grain 5 may have a conical configuration similar tothat at the downstream end shown in FIG. 2. Such a configuration willpermit total consumption of the fuel grain. As used herein, the termconical is not used in its strict geometric sense and does not requirethat the line generating the surface of revolution be a straight line.The line can be curved as shown to produce the configurations as shownin FIGS. 1 and 2.

Grain 5 is mounted within the member 2 with the injector tube 8positioned within the port of grain 5. The upstream end of injector tube8 is in communication with the interior of manifold 3 and the downstreamend of tube 8 is positioned in proximity to the point ofintersection ofthe port of grain 5 and the conical surface and is supplied with spraymeans 9 to produce a spray pattern of fluid that will flow over theconical surface of grain 5. Means are provided for applying a constantforce, acting in a downstream direction, to grain 5. Any electrical,mechanical, hydraulic or pneumatic device may be used to accomplish thisresult. A simple lightweight mechanism, utilizing constant force springssuch as those sold under the trade name of Neg-ator by the Hunter SpringDivision of Ametek Inc. is particularly useful. As shown, a plurality ofconstant force springs 10 are uniformly distributed about a supportplate 11, provided with a grain-receiving lip 12 and slidably mountedaround injector tube 8 by bearing 13. The ends of springs 10 are fixedto support member 2,

thereby maintaining a constant bias in the rearward direction on grain5.

A layer of thermal insulation 14 made from a refractory material such asfirebrick, for example, may be mounted on member 2 to protect the memberand the grain from the heat generated by the exhaust gases.

lf grain 5 and the oxidizer employed are hypergolic, no ignition meansare required. However, if the materials are not hypergolic, someignition means must be employed to heat the surface of grain 5 to atemperature sufficient to permit continued combustion. The conventionalmeans for accomplishing this is to supply a hot gas stream to thesurface just prior to the introduction of the fluid from injector 8.Normally the hot gas would be generated by burning a small portion of afuel and the oxidizer at the upstream end of the device. However, due tothe length of the injector tube 8, undesirable cooling of the gases andheat transfer to the grain 5 occur. Accordingly, a preferred ignitionsystem is illustrated wherein the hot gas stream is generated at thedownstream end of the injector. According to this system, the injectortube 8 is provided with a coaxial tube which terminates at itsdownstream end in plug member 9 provided with ports l6. Tube 15 isseparated from tube 8 by spider 17 formed from an electrical insulator.Similarly tube 8 is electrically insulated from manifold 3 by insulatingplug 18. The oxidizer is supplied to manifold 3 from a suitable source,not shown, through line 19 and variable flow valve 20, and fuel issupplied to tube 15 from a suitable source, not shown, through valve 21.Suitable fuels would include hydrogen and low mdlecular weighthydrocarbons such as methane, butane, or ethane, for example. Optionallya source of nitrogen or other purging or combustion extinguishing agentmay be connected to the manifold through line 22 and valve 23. A sourceof high voltage current 24 is connected to tube 15 and to manifold 3which is in electrically conducting relationship with injector tube 8. Aswitch 25 is inserted in the circuit so that a spark across the end ofplug 9 and tube 8 can be formed when desired. In operation the fuelgrain 5 would be mounted in the member 2 with the lip 7 restraining thedownstream motion of the grain 5. When operation is desired, valve 21and valve 20 would be opened to permit the fuel and oxidizer to flowthrough the system. Switch would then be closed to produce a sparkbetween member 9 and tube 8 which would ignite the initial mixture andestablish combustion on the inner conical surface of grain 5. Switch 25would then be opened and valve 21 would be closed. While this may beaccomplished mechanically suitable electrical sequencers would normallybe employed. As the combustion process occurs at the conical surfaceportion, the grain 5 will soften slightly and tend to assume theconfiguration shown in FIG. 1. The constant force springs continuallyurge the grain downstream, and if the oxidizer flow rate is varied, therate at which the grain moves downstream would automatically adjust tothe variation in the rate at which the grain is consumed. lfintermittent operation is required, valve 20 would be closed and toassure complete extinguishment, valve 23 would be opened to permitnitrogen to flow across the burning surface and extinguish any residualcombustion. Re-ignition could be accomplished in the manner previouslydescribed.

The fuel grain is preferably made from a material which does not charand which softens rather than melts on exposure to high heat fluxes.Suitable materials include carboxy-terminated polybutadiene polymers,polymethylmethacrylate, and cross-linked polystryene polymers. Additivessuch as carbon black or ionizable materials which can produce radiationat predetermined frequencies can be dispersed in the grain if desired.These polymers are particularly suitable for use with an oxygenoxidizer. Other oxidizers usable with various fuel components are knownto the art and would include N 0 F Cl fuming nitric acid andinterhalogen compounds, for example.

ln operation a grain of polymethylmethacrylate 19 inches long havinginternal diameter of 1.0 inches and an external diameter of 2.5 inchesand an internal conical surface 4 inches long was burned forapproximately 20 minutes using a flow rate of 0.0024 pounds per secondof oxygen at an O/F of approximately 1.2. A constant force of about 5pounds urged the grain against a restriction of 2.15 inches, l.D.

Throughout this disclosure the grain 5 has been considered to be formedof fuel material with the fluid being the oxidizer. This was done sincethe conventional hybrid rocket motor employs solid fuels and fluidoxidizers, however, it is readily apparent that the grain 5 could bemade of an oxidizer material and the fluid component could be the fuel.in such a system, the material injected through line 15 wouldnecessarily have to be an oxidizer material.

While this invention has been described with respect to a specificembodiment thereof, it should not be construed as limited thereto.Various modifications may be made without departing from the scope ofthis invention which is limited only the following claims wherein:

We claim:

1. A long burning hybrid combustor for combusting a solid component anda fluid component comprising in combination: 1 i

a. a grain support member comprising a body provided with a hollowcylindrical port, the downstream portion of which is provided with meansrestricting the internal diameter of said port,

b. a hollow cylindrical grain formed from a said solid component, theinner surface of said grain being provided at the downstream end with aconical surface, said end being received in sliding relationship to saidcylindrical port and abutting said means restricting the internaldiameter of said port,

c. fluid supply means for supplying said fluid com-,

an inwardly extending annular lip on said grain support member.

3. The apparatus of claim 1 wherein said fluid supply means comprises aninjector tube coaxially mounted within said hollow cylindrical grain,said tube terminating at its downstream end in proximity to the concaveconical portion of said grain and being provided at said end with meansfor directing said fluid component at said conical surface.

4. The apparatus of claim 1 wherein said means for applying a force tosaid grain comprises means for applying a constant force to said grain.

5. The apparatus of claim 4 wherein said constant force means compriseconstant force spring means acting upon and uniformly disposed aboutsaid grain.

6. The apparatus of claim 1 wherein said hybrid combustor is nozzleless.

7. The apparatus of claim 1 further comprising means for varying theflow rate of said fluid component.

8. A long burning hybrid combustor for combusting a solid component anda fluid component comprising in combination:

a. a grain support member comprising a body profluid supply means forsupplying said fluid component to the conical surface of said grain,

. means for maintaining said fluid supply means and said grain supportmember in fixed spacial relationship, and

. means for applying a force to said grain, said force acting to urgesaid grain against said means for restricting the internal diameter ofsaid port whereby said grain will move in the downstream direction asthe grain is consumed by combustion.

1. A long burning hybrid combustor for combusting a solid component anda fluid component comprising in combination: a. a grain support membercomprising a body provided with a hollow cylindrical port, thedownstream portion of which is provided with means restricting theinternal diameter of said port, b. a hollow cylindrical grain formedfrom a said solid component, the inner surface of said grain beingprovided at the downstream end with a conical surface, said end beingreceived in sliding relationship to said cylindrical port and abuttingsaid means restricting the internal diameter of said port, c. fluidsupply means for supplying said fluid component to the conical surfaceof said grain, d. means for maintaining said fluid supply means and saidgrain support member in fixed spacial relationship, e. means forapplying a force to said grain, said force acting to urge said grainagainst said means for restricting the internal diameter of said portwhereby said grain will move in the downstream direction as the grain isconsumed by combustion, and means for supplying a combustionextinguishing agent to said conical surface.
 2. The apparatus of claim 1wherein said means restricting the internal diameter of said portcomprises an inwardly extending annular lip on said grain supportmember.
 3. The apparatus of claim 1 wherein said fluid supply meanscomprises an injector tube coaxially mounted within said hollowcylindrical grain, said tube terminating at its downstream end inproximity to the concave conical portion of said grain and beingprovided at said end with means for directing said fluid component atsaid conical surface.
 4. The apparatus of claim 1 wherein said means forapplying a force to said grain comprises means for applying a constantforce to said grain.
 5. The apparatus of claim 4 wherein said constantforce means comprise constant force spring means acting upon anduniformly disposed about said grain.
 6. The apparatus of claim 1 whereinsaid hybrid combustor is nozzleless.
 7. The apparatus of claim 1 furthercomprising means for varying the flow rate of said fluid component.
 8. Along burning hybrid combustor for combusting a solid component and afluid component comprising in combination: a. a grain support membercomprising a body provided with a hollow cylindrical port, thedownstream portion of which is provided with means restricting theinternal diameter of said port, b. a hollow cylindrical grain formedfrom said solid component, the inner surface of said grain beingprovided at the downstream end with a conical internal surface and atthe upstream end with a substantially similar conical external surface,said downstream end being received in a sliding relationship to saidcylindrical port and abutting said means restricting the internaldiameter of said port, c. fluid supPly means for supplying said fluidcomponent to the conical surface of said grain, d. means for maintainingsaid fluid supply means and said grain support member in fixed spacialrelationship, and e. means for applying a force to said grain, saidforce acting to urge said grain against said means for restricting theinternal diameter of said port whereby said grain will move in thedownstream direction as the grain is consumed by combustion.